The transfer of liquid metal, in particular liquid aluminum, into molds to make castings is usually carried out by simply pouring under gravity. There are a number of severe disadvantages to this technique, in particular, the entrainment of air and oxides as the metal falls in a relatively uncontrolled way.
Counter-gravity is usually employed to avoid this problem. However, when making a series of castings using a counter-gravity system and a riser tube to supply metal to a mold, it has been found that if the metal is allowed to fall back down the riser tube during the process, oxides are immediately generated on the internal walls of the tube and subsequently carried into the next casting. The surface oxide exhibits the consistency of tissue paper and is easily folded into the melt, creating a folded film defect. In fact, the introduction of unwanted oxides into metal castings, especially in those applications using alloys having minimal or no silicon, is such a severe problem that often only the first casting is of an acceptable quality. All subsequent castings are unacceptable due to high oxide content.
To overcome the worst features of this method of mold filling, the so-called Low Pressure (LP) Casting Process was developed. In this technique the metal is held in a large bath or crucible, usually of at least 200-kg capacity of liquid metal, which is contained within a pressurizable enclosure known as a pressure vessel. The pressurization of this vessel with a low pressure (typically a small fraction such as 0.1 to 0.3 atmosphere) of air or other gas forces the liquid up a riser tube and into the mold cavity which is mounted above the pressure vessel.
The LP Casting Process suffers from the refilling of the internal crucible or bath. The metal has to be introduced into the vessel via a small door, through which a kind of funnel is inserted to guide the liquid metal from a refilling ladle through the door opening and into the pressure vessel. The fall into the funnel, the turbulent flow through the funnel and the final fall into the residual melt all re-introduce air and oxides to the liquid metal, the very contaminants that the process seeks to avoid.
Additional control problems occur in the filling of the mold because of the large size of the casting unit. First, the large volume of gas above the melt is of course highly compressible, and thus gives rather “soft” or “spongy” control over the rate of filling. Second, the problem is compounded because of the large mass of metal in the furnace, which needs to be accelerated by the application of the gas pressure. The problem is akin to attempting to accelerate (and subsequently decelerate) a battering ram weighing 200 kg or more by pulling on a few weak elastic bands.
The so-called Cosworth Process was designed to avoid this problem by the provision of melting and holding furnaces for the liquid metal, usually aluminum, which were joined at a common level, so that the metal flowed from one to the other in a tranquil manner. The liquid is finally transferred into the mold cavity by uphill transfer, using an electromagnetic (EM) pump which is permanently immersed in the melt, and which takes its metal from beneath the liquid surface, and moves it up a riser tube into the mold cavity without moving parts.
The control over the rate of flow of the metal is improved because the working volume in the pump and its delivery pipe is only a few kg. However, the driving force is merely the linkage of lines of magnetic flux, resembling the elastic bands in the mechanical analogy, so that control is not as precise as might first be thought.
Although there are many advantages to the Cosworth solution, the EM pump is not without its problems:                (i) It is expensive in capital and running costs. The high maintenance costs mainly arise as a result of the special castable grade of refractory for the submerged sections of the pump. These require regular replacement by a skilled person. In addition, they are subject to occasional catastrophic failure giving the various types of EM pumps a poor reputation for reliability. The disappointing trustworthiness is compounded by their extreme complexity and delicacy.        (ii) The relatively narrow passageways in the pump are prone to blockage. This can occur gradually by accretion, or suddenly by a single piece of foreign material.        (iii) Occasional voltage fluctuations cause troublesome overflows when the system is operating with the metal at the standby (bias) level.        (iv) At low metallostatic heads, the application of full power to the pump to accelerate the metal as quickly as possible sometimes results in a constriction of flow inside the pump as a result of the electrical pinch effect at high current density. If the pinch completely interrupts the channel of liquid metal current arcing will occur, causing damage, and temporarily stalling the flow. The pump has difficulty in recovering from the condition during that particular casting, with the consequence that the casting is filled at too low a speed, and is thus defective.        
A number of attempts have been made to emulate the Cosworth Process using pneumatic dosing devices which are certainly capable of raising the liquid into the mold cavity. However, in general these attempts are impaired by the problem of turbulence during the filling of the pressurizable vessel, and by the large volume of the apparatus, thus suffering the twin problems of large mass to be accelerated and large compressible gas volume to effect this action.
One of the first inventions to answer these criticisms effectively is described in British Patent 1,171,295 applied for Nov. 25, 1965 by Reynolds and Coldrick. That invention provides a small pressure vessel that is lowered into a source of liquid metal. An opening at its base allows metal to enter. When levels inside and out are practically equalized, the base opening is closed. The small internal gas space above the enclosed liquid metal is now pressurized, forcing the metal up a riser tube and into the mold cavity. After the casting has solidified, the pressure in the pump can be allowed to fall back to atmospheric, allowing the metal to drain back down the riser tube. The base opening can be re-opened to refill the vessel, which is then ready for the next casting. The compact pneumatic pump has been proven to work well in service.
The only major problem in service when pumping liquid aluminum has been found to be the creation of oxides in the riser tube. These are created each time the melt rises and falls. Thus the riser tube may not only become blocked, but oxides which break free are carried into the casting and impair its quality, possibly resulting in the scrapping of the casting. As mentioned, this is a particular problem with low silicon melts.
In U.S. Pat. No. 6,103,182, the disclosure of which is incorporated herein by reference, an apparatus for dispensing liquid metal is disclosed in which the metal is held between castings in a dispensing riser tube at a “stand-by” level that is close to, or actually at, the top of the riser tube. This inhibits the formation of oxides in the tube and greatly reduces the presence of oxides in the final castings. While this apparatus solved the oxide problem, it is relatively complex and expensive to produce, calling for multiple chambers and seals to be placed within the apparatus. In addition, a problem occurs in that the relatively limited diameter of the riser tube allows the molten metal held therein to cool much more rapidly than does the molten metal in the pressure vessel.
Thus, an apparatus is needed for dispensing low silicon containing melts into a mold that inhibits the contamination of the castings with oxides, that is mechanically relatively simple, that keeps the melt in the riser tube hot, and that is easy and inexpensive to operate and produce.